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Unusual Earthquakes
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While most recorded earthquakes are consistent with expectations
based on current understandings of plate tectonics, and with
simplifying assumptions about the earthquake source, there are
a number of very interesting exceptions. Earthquakes with
non-double couple focal mechanisms, or which contain a non-zero
isotropic component, may be the result of source processes other
than slip on a planar fault surface. Earthquakes which occur
outside of well-defined bands of seismicity, or which display
focal mechanisms at odds with the stress regimes we believe to
exist in a given region, may cause us to revise our understanding
of regional tectonics, or of deformation processes in general. The
Unusual Earthquakes research effort studies a small number of events
in detail, by a variety of methods, in an effort to gain a more
thorough understanding of source processes and tectonics which
differ from accepted standard models.

Some results
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We have analyzed the larger earthquakes in the
1997
Umbria-Marche (central Italy) sequence using
a new method of surface wave CMT analysis (Ekström
et al., 1998).
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We have analyzed in detail a sequence of
earthquakes in
Iceland which have highly non-double couple focal
mechanisms, and find that rupture on a volcanic cone-fault
explains our observations well (Nettles and Ekström,
1998).
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We re-analyzed the great 1970 Colombia earthquake using
the dataset of Dziewonski and Gilbert (1974),
taking advantage of theoretical and computational
advances in predicting the interaction of normal modes,
and find that considering splitting and coupling
induced by rotation, ellipticity and 3-D mantle
structure leads to the disappearance of the isotropic
precursor to this event (Russakoff et al., 1997).

Publications
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Moment tensor analysis of the central Italy earthquake
sequence of September-October 1997,
(Ekström, G. , A. Morelli, E. Boschi, and A. M. Dziewonski,
Geophys. Res. Lett., 25: 1971-1974, 1998.)
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A new analysis of the great 1970 Colombia earthquake
and its isotropic component,
(Russakoff, D., G. Ekström and J. Tromp, J. Geophys.
Res., 102: 20,423-20,434, 1997.)
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Faulting mechanism of anomalous earthquakes
near Bardarbunga Volcano, Iceland (Nettles, M. and G.
Ekström, J. Geophys. Res., 103: 17,973-17,983,
1998.)
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Anomalous earthquakes on volcano ring-fault structures
(Ekström, G., Earth Planet. Sci. Lett.,
128: 707-712, 1994.)
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Bering Sea earthquake of February 21, 1991: Active
faulting along the Bering Shelf edge (Abers, G., G. Ekström,
M.S. Marlow and E.L. Geist, J. Geophys. Res.,
98: 2155-2165,1993.)